The present invention relates to a wet-laid sheet material that is especially useful in forming compression molded composite plaques, said plaques being thermally and electrically conductive. More specifically, it relates to a wet-laid sheet material prepared from thermoplastic fibers, graphite particles, reinforcing fiber, and microglass fiber, and to composite plaques formed therefrom.
There currently exists a need for economical polymer systems having increased thermal and electrical conductivity capabilities. There have been various approaches in the past used to increase the thermal and electrical conductivity of polymer systems. One such approach involves increasing the conductivity of the polymer itself. However, the resultant intrinsically conductive polymer matrix has been found to be difficult and expensive to produce. Another approach involves coating the polymer with a thin layer of metal, such as silver or copper. Again, such an approach is not ideal because it is expensive (especially if preplating or priming is needed) and further because the metal coating could cause corrosion problems and could result in delamination due to thermal cycling. Finally, another approach involves adding conductive fillers to the polymer, such as carbon black, nickel-coated graphite fibers, nickel-coated glass fibers, stainless steel fibers, aluminum coated glass fibers, aluminum fibers, copper powder and flakes, and aluminum powder and flakes. However, such fillers can be expensive, difficult to process, subject to corrosion, and can cause resultant polymer system to be non-economical.
In the present invention, it was found that an economical polymer system could be made that had surprisingly good thermal and electrical conductivity. The system is a wet-laid sheet material made from a thermoplastic fiber, graphite particles for conductivity, reinforcing fibers for obtaining good physical properties, and microglass fiber to aid in retention of the graphite particles in the sheet materials. Wet-laid sheet materials are described in U.S. Pat. No. 5,134,016 as fiber reinforced porous sheets. However, this patent does not disclose sheets, or composite plaques made from them, that are thermally and electrically conductive, that contain graphite particles for conductivity, and that contain microglass fibers to aid in the retention of the graphite particles in the wet-laid sheet material.
The wet-laid sheet materials can be stacked and compression molded to form a composite plaque. The resultant plaque is found to have excellent transverse thermal conductivity and electrical conductivity, as shown by the examples herein. Comparable neat thermoplastic polymers, on average, have an average volume electrical resistivity of 10.sup.12 -10.sup.15, with some being even higher, and an average transverse thermal conductivity of about 0.2-0.3 W/mK. The materials of the present invention, as illustrated by the examples, have significantly improved conductivity values compared to comparable neat polymers.
The wet-laid sheet material of the present invention is useful in forming molded composite parts for use in applications requiring thermally and electrically conductive materials, such as heat sink applications (i.e., pump housings, power supplies for personal computers, light ballasts, encapsulation of electrical devices and parts thereof (including transformers), etc.), static dissipative or electromagnetic interference/radio frequency interference shielding applications, electrical grounding applications, electrical measuring devices (such as potentiometers), and electromagnetic radiation reflecting applications (e.g., antennae, etc.).